The formulation of effective detergent compositions presents a considerable challenge. Effective compositions are required to remove a variety of soils and stains from diverse substrates. In particular, the removal of greasy/oily soils quickly and efficiently can be problematic. For example, the removal of greasy food residues from dishware in hand dishwashing operations has become a particular challenge to the formulator. Modern dishwashing compositions are, in the main, formulated as aqueous liquids; accordingly, water-stable ingredients must be used. Moreover, such compositions come into prolonged contact with skin; therefore, they must be mild. Yet, mildness is difficult to achieve in an effective dishwashing product, since products which remove grease from dishware may also tend to remove the natural skin oils from the user's hands.
Various means have been suggested to enhance the grease and oil removal performance of detergent compositions. Grease-cutting nonionic surfactants have been employed, but some of these may be irritating to biological membranes. Some suggestions have been made to use nonconventional detergent surfactants in liquid compositions. Indeed, while a review of the literature would seem to indicate that a wide selection of surfactants is available to the detergent manufacturer, the reality is that many such materials are specialty chemicals which are not suitable in low unit cost items such as home-use detergent compositions. The fact remains that most home-use detergents still comprise one or more of the conventional ethoxylated nonionic and sulfated or sulfonated anionic surfactants, presumably due to economic considerations.
The challenge to the detergent manufacturer seeking improved grease/oil removal has been increased by various environmental factors. For example, some nonbiodegradable ingredients have fallen into disfavor. Effective phosphate builders have been banned by legislation in many countries. Moreover, many surfactants are often available only from nonrenewable resources such as petrochemicals. Accordingly, the detergent formulator is quite limited in the selection of surfactants which are effective cleaners, biodegradable and, to the extent possible, available from renewable resources such as natural fats and oils, rather than petrochemicals.
Considerable attention has lately been directed to nonionic surfactants which can be prepared using mainly renewable resources, such as fatty esters and sugars. One such class of surfactants includes the polyhydroxy fatty acid amides. Moreover, the combination of such nonionic surfactants with conventional anionic surfactants such as the alkyl sulfates, alkyl benzene sulfonates, alkyl ether sulfates, and the like has also been studied. Indeed, substantial success in the formulation of detergent compositions has recently been achieved using the N-alkyl polyhydroxy fatty acid amide surfactants. However, even these superior surfactants do suffer from some drawbacks. For example, their solubility is not as high as might be desired for optimal formulations. At high concentrations in water they can be difficult to handle and pump, so additives must be employed in manufacturing plants to control their viscosity. While quite compatible with anionic suffactants, their compatibility can be diminished substantially in the presence of water hardness cations. And, of course, there is always the objective to find new surfactants which lower interfacial tensions to an even greater degree than the N-alkyl polyhydroxy fatty acid amides in order to increase cleaning performance.
It has now been determined that the N-alkoxy polyhydroxy fatty acid amide surfactants surprisingly differ from their counterpart N-alkyl polyhydroxy fatty acid amide surfactants in several important and unexpected ways which are of considerable benefit to detergent formulators. The alkoxy-substituted polyhydroxy fatty acid amide compounds herein substantially reduce interfacial tensions, and thus provide for high cleaning performance in detergent compositions, even at low wash temperatures. The compounds herein exhibit more rapid dissolution in water than the corresponding N-alkyl polyhydroxy fatty acid amide surfactants, even at low temperatures (5.degree.-30.degree. C.). The high solubility of the compounds herein allows them to be formulated as modern concentrated detergent compositions. The compounds herein can be easily prepared as low viscosity, pumpable solutions (or melts) at concentrations as high as 70-100%, which allows them to be easily handled in the manufacturing plant. Moreover, the high solubility of the compounds herein makes them more compatible with calcium and magnesium cations, even in relatively concentrated compositions.
While it can thus be seen that the N-alkoxy polyhydroxy fatty acid amides provide substantial benefits, in the main they do tend to exhibit somewhat lower sudsing than their N-alkyl counterpart suffactants. However, users of the so-called "light-duty liquid" hand dishwashing compositions tend to equate product performance with suds height and persistence. Accordingly, modestly sudsing hand dishwashing compositions, while perhaps effective for their intended use, may be rejected by consumers based on their sub-optimal sudsing profile.
Succinctly stated, the invention herein is based on the discovery that use of specially selected "soap" materials can substantially enhance the grease and oil removal properties of detergent compositions which contain N-alkoxy polyhydroxy fatty acid amides. While not intending to be limited by theory, it appears that the inclusion of such soap materials into the present compositions substantially enhances their ability to rapidly lower the interfacial tension of aqueous washing liquors with greasy and oily soils. This substantial reduction of interfacial tension leads to what might be termed "spontaneous emulsification" of greasy and oily soils, thereby speeding removal from soiled surfaces and inhibiting the redeposition of the soils onto substrates. This phenomenon is particularly noteworthy in the case of hand dishwashing operations with greasy dishware.
It has further been determined that the use of common linear soaps does not provide optimum high sudsing, as is desired by the users of such compositions for hand dishwashing. Indeed, linear soaps are often used to diminish suds levels in certain European fabric laundering detergents; accordingly, the use of conventional linear soaps in the current compositions is sub-optimal, inasmuch as sudsing can suffer. Moreover, some soaps tend to provide their best grease cutting performance at pH's in the alkaline range, whereas it is much more desirable to have hand dishwashing compositions formulated at near-neutrality.
By the present invention it has been determined that certain soaps, e.g., secondary alkyl carboxylates, not only provide a desired additional lowering of interfacial tension, with its attendant increase in grease removal performance, but also, and importantly, allow the formulation of reasonably high sudsing liquid compositions which contain the aforesaid desirable N-alkoxy- polyhydroxy fatty acid amide surfactants, and which are stable and homogeneous. The inclusion of calcium ions in such compositions still further enhances the lowering of interfacial tension, and thus still further enhances grease removal performance. Moreover, the sudsing of such compositions can be increased even further by the addition of magnesium ions. These special benefits can be achieved at neutral pH, which enhances mildness and avoids the need for costly buffering chemicals. The overall unexpected improvements in performance and aesthetic qualities, especially sudsing, are described in more detail hereinafter.